Radio waves are electromagnetic waves that can be used in wireless communication. The frequencies of these waves serve as physical communication channels. The radio frequency (RF) spectrum has a finite range of frequencies, and thus a limited number of channels. In the United States, for example, the Federal Communications Commission (FCC) decides how the spectrum is allocated and what bands are used for what purpose. Communication signals on the same channel interfere, assuming the strengths of the signals are non-negligible due to transmission power and distance. Also, communication signals on adjacent channels may interfere with communications on the desired channel because of inadequate filtering, tuning or frequency control. Adjacent channel interference can increase with an increase in signal power in adjacent channels.
Most countries of the world have allocated certain frequency spectrums for commercial use as “unlicensed” wireless bands. For example, the FCC has designated license-free bandwidth segments for industrial, scientific and medical (ISM) uses. Various commercial applications use this unlicensed bandwidth for short range wireless communication. Channels are not allocated within the license-free band. Commercial devices designed to operate in the license-free band are constrained to transmit using a relatively low power, which allows more commercial devices to use the unlicensed frequency bands by increasing the reuse of a frequency.
Furthermore, not only may a number of commercial devices be operating within a communication band, but also more than one wireless technology may co-exist within the communication band. For example, the 2.4 GHz band includes wireless LAN, Bluetooth and IEEE 802.15.4 technologies. Wireless LAN has three non-overlapping channels with a bandwidth of 22 MHz. Bluetooth technology has 79 channels each with a bandwidth of 1 MHz, Bluetooth low energy technology has 40 channels each with a bandwidth of 2 MHz, and IEEE 802.15.4 has 11 channels with a bandwidth of 5 MHz. Wireless communication devices attempt to select adequate channels through which to communicate within this environment. For example, adaptive frequency selection may be used to select adequate channels based on both RSSI (Received Signal Strength Indication) noise measurements and based on retransmission attempts. Bluetooth has used adaptive frequency hopping (AFH) techniques to use only channels that do not have interference by measuring RSSI of unused channels and by keeping track of retransmissions on active channels. For example, PER (Packet Error Rate) may be used to assess the link quality of a communication channel. However, with AFH, a degraded signal is received before the channel is determined bad and removed from the channel map.
One challenge with RF communication is multipath propagation of signals in which radio signals reach the receiving antenna by two or more paths. The overall signal at the radio receiver is the sum of the signals being received. As multipath signals have different path lengths, the signals will add and subtract from the total dependent upon their relative phases. The effects of multipath include constructive and destructive interference and phase shifting of the signal, which may be perceived as multipath fading of RF signals. A mobile wireless device, particularly within a building, may encounter a dynamically-changing environment of multipath propagation as the device changes locations and orientations, and as objects and people also move within this environment. For example, an RF communication device typically worn by a human experiences dynamic multipath fading that is dynamic relative to the movements of a human being. These movements are far too fast for current adaptive frequency hopping techniques.
Accordingly, there is a need in the art for improving wireless communication with multipath propagation.